1. Field of the Invention
The invention relates to a temperature sensing circuit, more particularly to a temperature sensing circuit having relatively high accuracy that is not easily affected by component manufacturing tolerances.
2. Description of the Related Art
In recent years, rapid developments in integrated circuit technology have reached the stage where a single-packaged chip may contain millions of transistors. As such, when an integrated circuit configured with a large number of transistors operates at a clock rate of several hundred MHz, the amount of heat dissipated will be enormous to the extent that the operating temperature may exceed 100 degrees centigrade.
Due to the change in temperature, all components in the chip will be adversely affected, since temperature and conductivity have an inversely proportional relationship. Therefore, when temperature rises, the electrical characteristics of components will change accordingly. The most evident effect is that operating speed and overall efficiency are reduced.
Referring to FIG. 1, a conventional temperature sensing circuit 7 is shown to comprise a current mirror 71 and a Widlar current source 72 coupled thereto. As known to those skilled in the art, by matching transistors in the current mirror 71, the temperature sensing circuit 7 will have equal currents I1, I2, I3, i.e., I1==I2=I3. When the transistor (Q2) of the Widlar current source 72 operates in the forward active region, the current (I2) flowing through the transistor (Q2) will be:                               I          2                =                              1                          R              13                                ⁢                      V            T                    ⁢                      ln            ⁡                          (              n              )                                                          (        1        )            
wherein n is the emitter-base junction ratio between the transistor (Q2) and the transistor (Q1), and the thermal voltage (VT) is equal to 26 mVxc2x7T/300xc2x0 K. Since the voltage (VTEMP)=I3xc3x97R14=I2xc3x97R13, the following equation can be obtained:                                                                         V                TEMP                            =                                                                    R                    14                                                        R                    13                                                  ⁢                                  V                  T                                ⁢                                  ln                  ⁡                                      (                    n                    )                                                                                                                          =                                                                    R                    14                                                        R                    13                                                  ⁢                                  (                                                            26                      ⁢                                              mV                        ·                        T                                                                                    300                      ⁢                      xc2x0                      ⁢                                              xe2x80x83                                            ⁢                      K                                                        )                                ⁢                                  ln                  ⁡                                      (                    n                    )                                                                                                          (        2        )            
Therefore, the amount of change in the voltage (VTEMP) is determined by the values of n and (R14/R13). In order to facilitate monitoring of the temperature change, it is preferable that when the temperature rises by 1xc2x0 K, the voltage (VTEMP) rises by 1 mV accordingly. In other words,. (R14/RL3)xc2x7(26)xc2x7ln(n) must have a value of about 300. The relevant parameters in wide use to meet the above condition are: n=4, R13=3.6 K, and R14=30 K. By substituting these parameters into equation (2), the following equation can be obtained:                               V          TEMP                =                  300          ⁢          mV          xc3x97                      T                          300              ⁢              xc2x0              ⁢                              xe2x80x83                            ⁢              K                                                          (        3        )            
Therefore, when the temperature rises by 1xc2x0 K, the voltage (VTEMP) rises by 1 mV. As such, when the temperature sensing circuit 7 is coupled to a main circuit (not shown) the operating temperature of the main circuit can be monitored by observing the voltage (VTEMP) from the temperature sensing circuit 7 so that thermal protection of the main circuit can be activated when appropriate.
However, the foregoing analysis was made under ideal conditions in practice, due to manufacturing constraints, the actual output of the temperature sensing circuit 7 usually differs from the original design. It is noted that the accuracy of the voltage (VTEMP) depends on the actual values of n and (R14/R13) Therefore, during manufacturing, if a lower value of (R14/R13) is desired, a higher value of n must be provided for compensation. For example, if (R14/R13) is set as 2, the value of n must be as high as 320 to satisfy the condition that when the temperature rises by 1xc2x0 K, the voltage (VTEMP) rises by 1 mV. Nevertheless, the value of n is determined by the physical characteristics of the transistors (Q2) and (Q1) and cannot be adjusted. If manufacture of the transistors (Q2) and (Q1) is based simply on the calculated values, the outcome will be a mismatch in the current gains of the transistors (Q2) and (Q1), thereby resulting in failure of the temperature sensing circuit 7 to operate normally and inability of the temperature sensing circuit 7 to serve the purpose of temperature measuring. Thus, to ensure the accuracy of the characteristic curve of the circuit, a value smaller than 10 is usually adopted for n. This introduces another design problem since the value of (R14/R13) must be correspondingly increased to satisfy the aforesaid requirement. However, in view of manufacturing constraints, it is known that the resistance values of resistors cannot be accurately controlled. Due to the requirement of a high resistance ratio, the resultant error tends to be too high. As such, the measured result of the temperature sensing circuit 7 is imprecise.
Therefore, the main object of the present invention is to provide a temperature sensing circuit having relatively high accuracy that is not easily affected by component manufacturing tolerances.
Accordingly, a temperature sensing circuit of this invention comprises a current source for providing a bias current, a first transistor pair coupled to the current source, and a cascaded set of second transistor pairs.
The first transistor pair includes first and second transistors. The first transistor has a first collector, a first base coupled to the first collector, and a first emitter. The second transistor has a second collector, a second base coupled to the first base of the first transistor, and a second emitter. The first transistor pair has a first emitter-base junction ratio between the first transistor and the second transistor.
A first one of the second transistor pairs in the cascaded set is coupled to the first transistor pair. Each of the second transistor pairs includes third and fourth transistors. The third transistor has a third collector, a third base and a third emitter. The fourth transistor has a fourth collector coupled to the third base of the third transistor, a fourth base coupled to the third collector of the third transistor, and a fourth emitter. The third and fourth emitters of the third and fourth transistors of each of the first through a second to a last one of the second transistor pairs in the cascaded set are coupled to the third and fourth collectors of an adjacent one of the second transistor pairs in the cascaded set, respectively. Each of the second transistor pairs has a respective second emitter-base junction ratio between the third transistor and the fourth transistor thereof. A voltage output, which corresponds to temperature sensed by the temperature sensing circuit and which is a function of the first emitter-base junction ratio of the first transistor pair and the second emitter-base junction ratios of the second transistor pairs, is obtained from the third and fourth emitters of the last one of the second transistor pairs in the cascaded set.